Find/detect for first match in Ruby - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Find the first even number in an array

What is the output of this Ruby code that finds the first even number in the array?

Ruby

numbers = [1, 3, 5, 8, 10]
result = numbers.find { |n| n.even? }
puts result

A nil

B 10

C 8

D 1

💡 Hint

Remember, find returns the first element matching the condition.

The method find returns the first element for which the block returns true. Here, 8 is the first even number.

❓ Predict Output

intermediate

Detect the first string longer than 4 characters

What does this Ruby code print when detecting the first string longer than 4 characters?

Ruby

words = ['cat', 'dog', 'horse', 'elephant']
result = words.detect { |w| w.length > 4 }
puts result

A horse

B dog

C elephant

D nil

💡 Hint

Check the length of each word in order.

The first word longer than 4 characters is 'horse'.

❓ Predict Output

advanced

Find first hash with value greater than 10

What is the output of this Ruby code that finds the first hash with a value greater than 10?

Ruby

items = [{a: 5}, {b: 15}, {c: 8}]
result = items.find { |h| h.values.first > 10 }
puts result

A {:a=>5}

B {:c=>8}

C nil

D {:b=>15}

💡 Hint

Look at the values inside each hash.

The first hash with a value greater than 10 is {:b=>15}.

❓ Predict Output

advanced

Detect first number divisible by 7

What does this Ruby code output when detecting the first number divisible by 7?

Ruby

nums = [10, 14, 21, 28]
result = nums.detect { |n| n % 7 == 0 }
puts result

A 21

B 14

C 28

D 10

💡 Hint

Check each number in order for divisibility by 7.

14 is the first number divisible by 7 in the array.

🧠 Conceptual

expert

Understanding find vs detect in Ruby

Which statement is true about the Ruby methods find and detect?

A <code>find</code> and <code>detect</code> are aliases and behave identically.

B <code>find</code> returns the first match, <code>detect</code> returns all matches.

C <code>find</code> only works on arrays, <code>detect</code> only on hashes.

D <code>detect</code> is deprecated and should not be used.

💡 Hint

Check Ruby documentation for these methods.

In Ruby, find and detect are exactly the same method with two names.

Practice

1.

What does the Ruby method find (also known as detect) do when called on an array?

easy

A. It removes the first element that matches the condition in the block.

B. It returns all elements that match the condition in the block.

C. It returns the first element that matches the condition in the block.

D. It sorts the array based on the block's condition.

Solution

1. Step 1: Understand the purpose of find

   The find method searches through the array elements one by one.

2. Step 2: Identify what find returns

   It returns the first element for which the block condition is true, then stops searching.

3. Final Answer:

   It returns the first element that matches the condition in the block. -> Option C

4. Quick Check:

   find = first matching element [OK]

Hint: Remember: find returns only the first match, not all matches [OK]

Common Mistakes:

• Thinking find returns all matches
• Confusing find with methods that modify the array
• Assuming find sorts the array

2.

Which of the following is the correct syntax to find the first even number in an array nums using find?

nums = [1, 3, 4, 6]

easy

A. nums.find { |n| n.even? }

B. nums.find(n.even?)

C. nums.find => n.even?

D. nums.find |n| n.even?

Solution

1. Step 1: Recall the block syntax for find

   The block must be passed with curly braces or do-end and use a block variable with pipes.

2. Step 2: Check each option's syntax

   nums.find { |n| n.even? } uses correct block syntax: { |n| n.even? }. Others have syntax errors.

3. Final Answer:

   nums.find { |n| n.even? } -> Option A

4. Quick Check:

   Correct block syntax = nums.find { |n| n.even? } [OK]

Hint: Use curly braces and pipes for blocks with find [OK]

Common Mistakes:

• Missing pipes around block variable
• Passing condition without a block
• Using wrong arrow or syntax for block

3.

What is the output of the following Ruby code?

words = ["apple", "banana", "cherry", "date"]
result = words.find { |w| w.length > 5 }
puts result

medium

A. "cherry"

B. nil

C. "apple"

D. "banana"

Solution

1. Step 1: Check each word's length

   "apple" (5), "banana" (6), "cherry" (6), "date" (4).

2. Step 2: Find first word with length > 5

   "banana" is the first word longer than 5 characters.

3. Final Answer:

   "banana" -> Option D

4. Quick Check:

   First word with length > 5 = "banana" [OK]

Hint: Check elements in order; find stops at first match [OK]

Common Mistakes:

• Choosing the last matching word instead of first
• Confusing length condition
• Assuming find returns all matches

4.

Identify the error in this Ruby code that tries to find the first negative number:

numbers = [3, 5, -2, 7]
result = numbers.find do |n|
  n < 0
end
puts result

medium

A. There is no error; the code works correctly.

B. The variable n is not defined in the block.

C. The block syntax is incorrect; should use curly braces.

D. The method find does not accept a block.

Solution

1. Step 1: Check block syntax

   The do...end block with variable n is valid syntax for find.

2. Step 2: Confirm method usage

   find accepts a block and returns the first element matching n < 0.

3. Final Answer:

   There is no error; the code works correctly. -> Option A

4. Quick Check:

   Valid block syntax and method usage = There is no error; the code works correctly. [OK]

Hint: Remember: do-end blocks are valid for find [OK]

Common Mistakes:

• Thinking only curly braces work for blocks
• Assuming variable must be declared outside block
• Believing find can't take blocks

5.

You have an array of hashes representing people:

people = [
  {name: "Alice", age: 18},
  {name: "Bob", age: 17},
  {name: "Carol", age: 22}
]

Which code finds the first person who is an adult (age 18 or older) and returns their name?

hard

A. people.detect { |p| p[:age] > 18 }[:name]

B. people.find { |p| p[:age] >= 18 }[:name]

C. people.find { |p| p[:age] > 18 }[:name]

D. people.detect { |p| p[:age] >= 18 }[:age]

Solution

1. Step 1: Understand the condition for adult

   Adult means age 18 or older, so condition is p[:age] >= 18.

2. Step 2: Find first matching person and get name

   Use find or detect with condition, then access [:name].

3. Step 3: Check options carefully

   people.find { |p| p[:age] >= 18 }[:name] uses correct condition and returns name. people.detect { |p| p[:age] >= 18 }[:age] returns age, not name. Options A and C use > 18, excluding age 18.

4. Final Answer:

   people.find { |p| p[:age] >= 18 }[:name] -> Option B

5. Quick Check:

   Correct condition and return name = people.find { |p| p[:age] >= 18 }[:name] [OK]

Hint: Use >= 18 to include age 18, then get [:name] [OK]

Common Mistakes:

• Using > 18 excludes age 18
• Returning age instead of name
• Confusing find and detect (both work but check return)